The present invention relates to protecting carbon/carbon (C/C) thermostructural composite materials against attack in an aggressive medium.
C/C composite materials are well known and they are used for their thermal properties (refractoriness of carbon) and in particular for the excellent mechanical behavior they present even at high temperature. Nevertheless, such materials present the drawback of being porous and of being corroded by any corrosive agent that is active with respect to the element carbon.
C/C composite materials are used in particular for forming parts that are to be subjected to high temperatures, such as the nozzles of rocket engines, airplane brake disks, the walls of nuclear reactors (fission, fusion), atmospheric reentry devices, etc. While such C/C composite material parts are in use, it is very important to prevent the carbon that is exposed at the “accessible surface” of the part from interacting with oxidizing species, and above all with the corrosive agents that are present. The term “accessible surface” is used to mean all of the outside surface of the part including the surfaces of pores that are inside the material and that open to the outside, i.e. including the pores that are accessible to the corrosive medium from the outside.
In order to protect this accessible surface of the material, one well-known solution consists in causing a molten metal to react with the carbon that is present so as to obtain a carbide layer. The metal is selected so that it generates a carbide that withstands the corrosive medium under consideration and that is stable at high temperature, such as silicon carbide, which is obtained by causing the carbon of the material to react with molten silicon. In order to protect the C/C material, it is desired more particularly to form carbides that are highly refractory, also referred to as ultra-refractory carbides, i.e. carbides that present melting temperatures higher than 2000° C., such as titanium carbide (TiC), zirconium carbide (ZrC), niobium carbide (NbC), and hafnium carbide (HfC). These carbides are obtained from the corresponding metals raised to their melting temperatures, e.g. by using the well-known method of reactive melt infiltration (RMI).
FIGS. 1A and 1B show the result that is obtained after infiltrating a C/C composite material with molten zirconium. As can be seen on the portion of material shown in FIGS. 1A and 1B, a layer of zirconium carbide (ZrC) is indeed obtained on the carbon surface that is accessible to the molten zirconium as a result of the reaction Zr+C→ZrC. By way of example, the document by L. M. Adelsberg et al., “Kinetics of the zirconium-carbon reaction at temperatures above 2000° C.”, Transactions of the Metallurgical Society of AIME, 1966, No. 236, pp. 972-977, describes the reaction of zirconium with carbon at about 2000° C., with zirconium carbide being formed.
The metallographic sections shown in FIGS. 1A and 1B show that there is a bonding defect between the layer of zirconium carbide formed on the carbon of the material. The decohesion (corresponding to the black zones between the carbon and the ZrC in FIGS. 1A and 1B) constitutes a preferred passage for one or more corrosive agents, and consequently for the carbon of the C/C material part being attacked during subsequent use thereof.
Methods have been developed that attempt to connect the carbide layer with the carbon of the material. In particular, document US 2004/0207133 proposes performing an initial RMI reactive infiltration using a refractory metal to form the desired carbide, and subsequently performing a second RMI reactive infiltration operation with silicon on its own in order to fill the spaces that result from the decohesion with a secondary layer of silicon carbide obtained by reaction between the molten silicon and the accessible carbon. Nevertheless, apart from the fact that that method requires second heat treatment of the material at high temperature, thereby giving rise to extra fabrication costs, the Applicant has found that even when forming such an SiC layer after the initial carbide has been formed, it is not possible to obtain good bonding between the various layers, in particular in terms of structural and thermal continuity of the bonding.
The Applicant has observed that the decohesion between the carbon of the composite material and the carbide occurs during the cooling of the molten metal deposited on and in the material. The difference in thermal expansion coefficients between the molten metal and the composite material may be at least partially responsible for this decohesion.